1. Field of the Invention
The present invention relates to an ignition control device for an internal combustion engine, for controlling an ignition device capable of performing ignition operation a plurality of times in one combustion cycle.
2. Description of the Related Art
A conventional ignition control device for an internal combustion engine of this type is disclosed in Japanese Laid-Open Patent Publication No. H11-37030. This ignition control device detects the rotational speed of the engine and load thereon (opening degree of a throttle vale, for example), and increases the number of times of discharge (ignition operation) of a spark plug in one combustion cycle as the detected rotational speed is lower and also as the detected load is lower. The ignition control device is configured by taking into account a fact that atomization of injected fuel is liable to be insufficient, since the temperatures of a piston and a cylinder are relatively low when both the rotational speed of the engine and the load thereon are low. The ignition performance of the ignition control device is improved by increasing the number of times of discharge and ignition energy.
The above-described conventional ignition control device is based on preconditions that the increase in the number of times of discharge of the spark plug directly leads to improvement in the ignition performance. However, it has been found that the relationship between the number of times of discharge and the advantageous effect of improvement in the ignition performance is not necessarily uniform, and particularly, the relationship largely depends on a flowing state of a mixture in the cylinder. For example, in such a case where in-cylinder flow for causing a mixture to forcibly flow in the cylinder is performed, it is required to ignite a mixture having a very high flow velocity, depending on a set ignition timing. In this case, to ignite the mixture against the high flow velocity without flame blowoff, it is very effective to increase the number of times of discharge, whereas when the flow velocity of the mixture is low, the above advantageous effect can be hardly obtained even if the number of times of discharge is increased.
However, in the conventional ignition control device, when the condition that both the rotational speed of the engine and the load thereon are low is satisfied, the number of times of discharge of the spark plug is controlled to be increased irrespective of the flowing state of the mixture in the cylinder. Therefore, when the flow velocity of the mixture is low, the number of times of discharge of the spark plug is wastefully increased in the state in which the advantageous effect of improvement in the ignition performance can be hardly obtained, whereby erosion of the spark plug proceeds to shorten the service life thereof and increase power consumption.